1. Field of the Invention
This invention relates to an electronic musical instrument, and more particularly to an electronic musical instrument in which conversion of a musical waveshape under the control of a tone control device does not exert any influence on the musical waveshape generation and which has a rapid response to the musical waveshape conversion. More in particular, the invention is directed towards an electronic musical instrument of the digital system in which required musical waveshape data are computed by a tone control device to obtain a complex waveshape and the complex waveshape is read out at a read-out frequency provided by closure of an instrument key-board switch to generate the selected musical note.
2. Description of the Prior Art
In an apparatus having a large number of key switches such as the keyboard of an electronic musical instrument, direct connection of the key switches to desired circuits for transmitting key information to selected ones of them as required will involve an enormous amount of wiring, and hence is very uneconomical. Further, it is difficult to employ the semiconductor integrated circuit techniques because of too large a number of pins used.
In view of the above, there has recently been considered a system which scans all of the keyboard switches in a predetermined period of time and generates pulses at the moments corresponding to depressed ones of the key switches, thereby to save the connections between the key switches and circuits. This type of system that has usually been employed is, for example, a key code multiplex in which information of depressed key switches detected by scanning all of the key switches on a time shared basis is transmitted in the form of a TDM (Time Division Modulation) or PCM (Pulse Code Modulation) signal. With such a system, however, since the time for scanning all of the key switches is fixed, there will be a waste of time in the case where the number of depressed key switches is small.
In a play of an ordinary keyboard musical instrument, a maximum number of key switches simultaneously depressed with both hands and feet is eleven. If each block of key switches is considered to cover one octave, it is impossible to depress the key switches of two or more octaves with one hand, so that a maximum number of blocks simultaneously occupied is five. Accordingly, the key switches are divided into a plurality of blocks and these blocks are scanned and when there is even one depressed key switch, scanning is stopped at that block and the depressed key switch is detected. Since scanning skips over those of the blocks in which no key switches are depressed, one scanning time for obtaining information of the depressed key switches can be shortened.
The required musical waveshape data are calculated by a waveshape calculator included in a tone control device to obtain a complex waveshape, which is read out by a read-out frequency produced by closure of a key switch, thereby to generate the selected musical note. In a conventional system for producing such a tone signal, the complex musical waveshape calculated by the waveshape calculator based on the musical waveshape data is written in a buffer register, from which it is written simultaneously in the note registers corresponding to channels CH.sub.1 to CH.sub.n in a short time independently of the period of the produced note. Consequently, during key depression, a noise is generated by the waveshape calculation, exerting a bad influence on the waveshape generation controlled by the tone control device.